The polymer adsorption regimes that correspond to very diluted bulk concentration and diluted to semidiluted surface concentration (on the solid substrate) were studied both theoretically and experimentally. The extremal behavior of the adsorbed polymer layer was observed experimentally with two independent methods : (1) contact angle measurements of adsorbed polymer films of poly(nonyl acrylate) and poly(methyl methacrylate) (PMMA) on the surface of aluminum foil and (2) electrochemical reduction of K3Fe(CN)(6) on the surface of a platinum electrode covered by the adsorbed polymer layer. As recently reported (Adsorp. Sci. Technol. 1996, 14, 251), the same behavior was observed on the surface of ZnO powder. The extremal behavior was found at polymer concentrations of about 0.001% in the bulk, or 1-5% from the plateau adsorption value. We suggest these data can be explained by the sharp change of conformation (and fraction of bonded units per chain) of adsorbed macromolecules when the transition from the diluted to semidiluted regime occurred. The calculations performed on the basis of the partition function combined with an associative integral equation predict the extremum in the surface coverage due to specific dependence of the intramolecular correlation function on the volume concentration of polymer.